In the development of gas turbines, both, an increased cycle performance and reduced pollutant emissions are key targets to minimize the environmental impact as well as maximize the economic benefit. In order to increase gas turbine efficiency, it is important that both the distribution of the air exiting the compressor and the distribution of the hot gases exiting the combustor are optimized, i.e. that the working fluid experiences the smallest possible pressure drop before it starts to expand in the turbine.
The before goals may be achieved inter alia by arranging a cooling path for the combustor walls and the burner air path in parallel which is illustrated in FIG. 2a. Concerning FIG. 2a a rough sketch of a burner arrangement is illustrated comprising a plenum 1 which is fluidly connected with a compressor stage of an stationary gas turbine (not shown), so that the volume of the plenum 1 is filled with compressed air 2 under a prevailing pressure p1. The plenum 1 encapsulates a burner arrangement comprising a burner section 3 which is surrounded by a burner hood 4 having means for fuel injections 5, means for air supply 6 and means for generating a fuel/air mixture (not shown) which is ignited inside a combustion chamber 7 following downstream of the burner section 3. Hot gases 8 which are produces inside said combustion chamber 7 exiting said burner arrangement directly into a turbine (not shown) for performing work by expanding. To avoid any thermal overloading of the burner arrangement especially of the combustor, the combustor wall provides a combustor liner containing an interspace 9 into which compressed air 2 form the plenum 1 respectively form the compressor enters the interspace 9 for cooling purpose. The interspace 9 represents a cooling air path to cool the combustor walls. The cooling air emits the cooling air path and enters the combustion chamber directly. Also a part of compressed air inside the plenum 1 enters the burner section 3 via the means for air supply in form of access openings 6 inside the burner hood 4 for mixing with fuel which is injected by the injection means 5 for generating an ignitable fuel/air mixture 11.
The drawback of such a system is however the fact that not all of the air which is fed by the compressor inside the plenum takes part into the combustion, therefore a higher flame temperature is achieved for the same hot gas temperature, with the consequence of higher NOx emissions. Alternatively, if the layout is targeting lower NOx, the hot gas temperature has to be reduced, thereby negatively impacting engine efficiency.
An alternative System is often used, in which the cooling and burner air paths are connected in series, see FIG. 2 b. FIG. 2 contains same reference signs which label components already explained in FIG. 2a so that for avoiding repetitions these components are not explained again. Here, the cooling path for the combustor which is the interspace 9 is fluidly connected with the burner section 3, so that the cooling air enters the burner via means for air supply 6 to be mixed with fuel for generating the fuel/air mixture 11.
This has the advantage that the whole air massflow takes part into the combustion, therefore emissions are minimized, however the overall pressure loss may be higher in this case, and therefore efficiency is lower. With such a layout, the pressure loss of the cooling path can optionally be reduced by bypassing some of the air 11 directly from the plenum 1 to the burner hood 4. The bypass air 11 is, however, still experiencing a pressure loss and thereby providing no additional benefit.